1. Field of the Invention
The present invention relates to a filter element for removing minute particles from a raw fluid used in the industries of semiconductors, pharmaceuticals, chemicals, etc., such as a strongly acidic or basic fluid having high corrosiveness or reactivity, particularly an active fluid including an organic solvent, for example, also to a method for the manufacture of the filter element and further to a microfiltration filter having the filter element.
2. Description of the Prior Art
As for filter media, there have heretofore been known filter paper, filter cloth, filter net, nonwoven fabric and sintered articles. When the diameter of minute particles in a raw fluid is on the order of microns or submicrons falling within a so-called micro-filtration region, porous membranes made of various polymers have been used. Since the porous membranes are thin films having a thickness in the range of from tens of microns to hundreds of microns and have a high porosity, they are very brittle and, when being used per se, difficult to resist high pressure of filtration. For this reason, the porous membranes have been retained on a meshy support member in order to prevent their deformation or breakage and to form flow paths for the cleaned fluid. In addition, the porous membranes exhibit high precision of filtration, whereas they exhibit high resistance of filtration per unit area and are liable to clog. Therefore, the porous membranes are required to have a large area in comparison with the area of the aforementioned filter media ordinarily used.
In view of this and the recent tendency to miniaturization of a filter having a porous membrane, various constructions of such filters have been proposed.
For example, FIG. 1 illustrates a prior art pleat cartridge filter (hereinafter referred to simply as the "pleat type filter"), in which a porous membrane 1 and support members 2 intervening the porous membrane 2 therebetween are pleated and folded up. The folded up state of the porous membrane 1 and support members 2 is retained by an outer protective cylinder 3 and a center core 4. The opposite ends of the protective cylinder 3 are sealed up by seal members 5. The pleat type filter having the construction described above is disadvantageous in that it cannot resist high pressure of filtration, that the effective area of the porous membrane 1 is decreased due to the presence of the area for an adhesive agent, and that the loss of pressure of filtration becomes large because the adjacent portions of the pleatd and folded up porous membrane 1 are close to each other.
FIGS. 2 and 3 illustrate prior art filters of a type in which a plurality of unit members each having a support member laminated with a porous membrane are piled up (hereinafter referred to simply as the "pileup type filter"). In FIG. 2, a support member 10 having a plurality of ribs 8 for forming fluid paths for the cleaned fluid is laminated with a porous membrane 6 which is supported on the ribs 8 and attached to the support member 10 at both the outer periphery 12 of the support member 10 and the peripheral edge 14 of a cleaned fluid takeout opening, thereby forming a unit member 18, and a plurality of unit members 18 are piled up vertically at regular intervals in a coupled state through boss portions 16. In FIG. 3, reference numeral 7 designates a porous membrane, 9 a rib, 11 a support member, 13 the outer periphery of the support member 11, 15 the peripheral edge of a cleaned fluid takeout opening, 17 a butt joint portion, and 19 a unit member. The pileup type filter shown in FIG. 3 has a construction substantially the same as that of the pileup type filter shown in FIG. 2 and has a plurality of unit members 19 piled up vertically at regular intervals in a coupled state through the butt joint portions 17. These pileup type filters are free from the aforementioned disadvantages of the pleat type filter.
The filters using a porous membrane have been developed, irrespective of the type thereof, in order to filtrate a realtively inactive fluid, such as water, air, nitrogen, etc. in the pharmaceutical and food industries and, therefore, the porous membrane is made of cellulosic polymers including cellulose acetate and cellulose nitrate, polyamide, polysulfone, polyvinyl chloride, polymethyl methacrylate, polyvinyl alcohol, polycarbonate, polyethylene or polyvinylidene fluoride, for example. The support member is molded of polystyrene, polycarbonate, polycarylonitrile, polysulfone or polypropylene, for example. Other members with which a fluid will come into contact, such as a seal member, are formed of natural or synthetic rubber, polyuurethane or epoxy resin, for example.
With the progress of the industries requiring complete elimination of foreign matter, represented by the semiconductor industry, minute particles in a highly active fluid including a strongly acidic or basic fluid exhibiting high corrosiveness, a fluid exhibiting high reactivity and an organic solvent have been regarded as foreign matter. Recently, therefore, there is an increasing demand for the development of filters capable of filtrating such a highly active fluids as described above in addition to a comparatively inactive fluid including water and air. However, filters made of any of the widely used polymers enumerated above cannot sufficiently deal with such an active fluid from the standpoint of resistance to chemicals and solvents. Particularly when fluids used in the etching and epitaxy processes in the manufacture of semiconductors are to be filtrated, filters have to be made of a chemically stable material. For example, fluorine resins and fluorine resin copolymers such as PTFE, PFA, EPE, FEP, ETFE, PCTFE and ECTFE can be practically used effectively as a material for filters. Thus, there is an increasing demand for the development of small-sized highly reliable filters having a porous membrane, a support member, a seal member and any other fluid-contacting member formed of a fluorine resin or fluorine resin copolymer so that they do not suffer from any disadvantage when being brought into contact with a fluid.
In the meantime, it is one of the most important techniques for producing a filter element of any type of filter how the flow paths for a raw fluid and for a cleaned fluid are separated from each other with exactitude in order to prevent the raw fluid from leaking and mixing with the cleaned fluid. That is to say, means for sealing a porous membrane with exactitude is important in the case of the pleat type filters and, in the case of the pileup type filters, means for sealing a porous membrane and a support member with exactitude is important.
As for the sealing means usable for constructing a filter element formed of any of the aforementioned widely used polymers for filtrating an ordinary inactive fluid, there can be adopted any one of the conventional general resin-sealing methods, such as the hot press method, ultrasonic sealing method solvent sealing method and method using adhesives or sealants, for example. However, these general resin-sealing methods are not effectively applicable to a filter element having all fluid-contacting members thereof including a porous membrane formed of a fluorine resin or fluorine resin copolymer for filtrating an active fluid because a fluorine resin or fluorine resin copolymer is characterized in that it has a high melting point, that it exhibits poor fluidity and low heat transmission property even at the melting point thereof, that it has a small coefficient of friction and that it is chemically inactive. Due to the physical and chemical properties of a fluorine resin or fluorine resin copolymer, it is very difficult to seal fluorine resins or fluorine resin copolymers together, or to seal a fluorine resin and a fluorine resin copolymer. In fact, there is little prior art disclosing an easy and reliable method for sealing them and no prior art disclosing an easy and reliable method for sealing a special portion of a specific shape, such as a portion of the pleat type filter to be sealed, portions of unit members constituting a filter element of the pileup type filter.
Therefore, it is considered that a thermal sealing method comprising the steps of heating surfaces of fluorine resins and/or fluorine resin copolymers to be sealed uniformly as much as possible to temperatures higher than their melting points and thereafter immediately, preferably simultaneously, contacting the surfaces under pressure is a sole and relable means. However, since they are inferior in heat transmission property, the surfaces thereof to be sealed must be heated directly. This will raise a problem on how heat is given to the surfaces. Indirect heat generating methods, such as ultrasonic sealing method, vibration sealing method and rotation sealing method, utilizing friction between the portions to be sealed will produce dust and will therefore be unsuitable as a sealing method of a filter requiring clearness of the filtrate. The surfaces to be sealed must be fused by a flame or by blowing a high-temperature gas and immediately thereafter, preferably simultaneously, pressed against each other.
Due to the recent tendency to miniaturization of a filter element of a pileup type filter, the thickness of a support member and the distance between adjacent support members must be made as small as possible. However, in Japanese Utility Model Public Disclosure No. 59-102111Japanese Patent Public Disclosures Nos. 56-129016, 58-98112 and 59-62323, it is very difficult to cause portions being sealed to adhere to each other under pressure by directly heating the portions from a gap between adjacent support members or from an opening of the support member in view of the shapes of the respectively disclosed support members. Therefore, it is necessary to adopt the second best method which comprises heating portions being sealed separately and causing the portions to adhere to each other under pressure as soon as possible after the heating step. However, very harsh conditions must be controlled in order to heat the portions being sealed uniformly without deforming the portions being sealed and without injuring porous membranes and portions having nothing to do with the sealing, and an exclusive special automatic machine is required in order to enhance the yield in a given process.
In view of the aforementioned difficulty, the piled up unit members 18 in FIG. 2 (Japanese Patent Public Discoslure No. 58-98111) are sealed not by heating but by the use of screws. With such mechanical sealing, it is difficult to ensure the sealing property.
In order to solve the aforementioned problems, the inventors have conducted various studies on conditions for sealing fluorine resins together and on shapes of the portions to be sealed. Generally, fluorine resins have a melting point in the range of 200 to 300 degrees centigrade which is higher than the melting points of the widely used resins, exhibit an inferior heat transmission property, and do not exhibit fluidity necessary for thermal adhesion even at tempertures higher than their respective melting points. For this reason, fluorine resins are inferior in thermal adhesion property even it they are of the same kind. Therefore, it has come to a conclusion that the thermal adhesion of fluorine resins is effected by heating the portions to be sealed at their surfaces, preferably to some depths thereof, at a predetermined temperature higher than the melting point to cause fluidity necessary for adhesion on the surfaces and thereafter immediately removing a heat source and contacting the surfaces under pressure and that more preferably the aforementioned heating is effected with the surfaces kept in contact with each other.
The support member for supporting thereon a porous membrane used in a planar state as a filter medium for a single layer filter or pileup type filler has a construction such that it has a plurality of concentric ribs (Japanese Patent Public Disclosure No. 56-129016) having the same width and spaced at regular intervals so as to support the whole of the filter medium and also has a plurality of slits communicating with an outflow port for a cleaned filtrate or has a construction such that it is formed of a net or a film having a plurality of bores communicating with an outflow port for a cleaned filtrate.
The drawbacks suffered by the conventional filter having the aforementioned support member with ribs will be described with reference to FIGS. 4(A) and 4(B).
Referring to FIG. 4(A), a support member 22 has a plurality of ribs 22a having a small width and having a large space left between the adjacent ones for the purpose of increasing the effective area of a membrane 21 to increase the flow rate of a cleaned filtrate. With this structure, however, the membrane 21 is liable to be flexed or damaged by the filtration pressure. When the membrane is flexed, it will come into intimate contact with the bottom of the support member 22 between the ribs 22a, thereby decreasing the flow rate of a cleaned filtrate.
Referring to FIG. 4(B), a support member 22 has a plurality of ribs 22a having a large width and having a small space left between the adjacent ones for the purpose of eliminating the drawbacks suffered by the support member of FIG. 4(A). With this structure, however, the effective area of a membrane 21 is decreased because the membrane comes into intimate contact with the large width of the ribs 22a.
As is clear from the comparison between the support members 22 shown in FIGS. 4(A) and 4(B), the width of and the space between the ribs 22a are determined by the strength of the membrane 21 and the loss of the effective area of the membrane 21 produced by the contact between itself and the ribs 22a must be considered as an inevitable consequence. In addition, the increase in effective area of a membrane will hinder the recent tendency to miniaturization of a filter. Furthermore, in the case where the height of spaces constituting flow paths for a cleaned filtrate passing through the membrane is constant, the larger the pressure loss the smaller the space areas, leading to an increase in resistance against the flow paths and a possibility of the flow paths being stopped up.
On the other hand, disposable pileup type filters have been proposed such as in Japanese Patent Public Disclosure No. 56-129016 and Japanese Utility Model Public Disclosure No. 59-102111, for example.
In the former Disclosure, as shown in FIG. 5, a support member 31 having a plurality of ribs 31a is prepared by injection molding and two porous membranes 32a and 32b of different shapes immersed in a solvent are attached to the opposite surfaces of the support member 31 respectively to form a unit member. A plurality of such unit members are piled one on top of another at regular intervals to constitute a disposable pileup type filter. Use of the two membranes of different shapes will increase the cost of parts and assembly cost. Use of a solvent will cause attachment of the solvent to undesirable portions of the membranes 32a, 32b and support member 31 and will also cause an adverse phenomenon, such as clogging of the membranes. Furthermore, the attachment of the membranes 32a and 32b over the entire surfaces of the ribs 31a will decrease the effective area of the membranes.
In the latter Disclosure, as shown in FIG. 6, a support member 36 is composed of upper and lower members 36a, 36b of different shapes each having a plurality of ribs. The two members 36a and 36b of different shapes are assembled with a prescribed space formed therebetween into the support member 36 having the upper and lower surfaces which are the same in shape. A pair of porous membranes 32 of the same shape immersed in a solvent are attached to the upper and lower surfaces of the support member 36 to form a unit member. A plurality of such unit members are piled one on top of another with a spacer intervening therebetween on the side of the flow path for a raw fluid. Thus, the assembly of a plurality of unit members into a disposable pileup type filter is cumbersome. While the filter shown in FIG. 5 uses two porous membrane 32a and 32b of different shapes, the filter shown in FIG. 6 uses two members 36a and 36b of different shapes constituting the support member 36. Therefore, the cost of parts and assembly cost will similarly be increased. The other drawbacks suffered by the filter shown in FIG. 5 still remain in the filter shown in FIG. 6.
In the micro-filtration field, there are hollow yarn type, coil type, pleat type and pileup type modules. In any of these modules, a filter element is of a cartridge and, when being integral with a housing for accommodating the filter element, it can serve as a filter. The material of which the housing is made is generally divided into metal and synthetic resin and is determined depending on various conditions, such as the kind, degree of activity, temperature, pressure, etc. of a fluid.
A filter belonging to the micro-filtration filter which has recently been used in the semiconductor industry requires its fluid flow paths to have smooth surfaces and also requires its housing to have a smooth inner surface. In addition, it is necessary to prevent minute particles of the material of a filter element from being scattered within the housing.
Referring to FIGS. 7, 8, 9(A) and 9(B), a fluid introduced into an inlet portion 41a of a housing 41 is filtered by filter elements 42 incoporated into the housing 41 and is taken out as a cleaned filtrate from an outlet portion 41b of the housing 41. Since the downstream side 42b of the filter elements 42 is sealed tightly by the use of an O-ring 43 (or a gasket, adhesive, etc.), the cleaned filtrate can be taken out of the outlet portion 41b without being contaminated by the raw fluid. On the other hand, the upstream side 42a of the filter elements 42 is not so tightly sealed. For this reason, the prior art shown in FIG. 7 has the upstream side 42a kept in the state of not in contact with the inner surface of the housing 41, that shown in FIG. 8 has a shock absorbing member 44 (e.g. a spring or rubber member) provided so as to enhance the tight seal construction on the downstream side 42b, and that shown in FIG. 9(A) or FIG. 9(B) has a ridge 45 or a projection 46 with a slit 46a provided at a position in the vicinity of the inlet portion 41a for the purpose of supporting thereon part of the filter elements 42 on the upstream side 42a.
In the case where the filter elements 42 on the upstream side 42a is not in contact with the inner surface of the housing 41, as illustrated in FIG. 7, there is a possibility of a seal mechanism on the upstream side 42a being damaged by external impact during the conveyance of the filter, or of the O-ring 43 being detached from the mounting portion. This will cause the filter to malfunction. In addition, there is a possibility of the O-ring 43 being detached form the mounting portion by the reverse pressure in use.
In the case where the shock absorbing member 44 is interposed between the upstream side 42a of the filter elements 42 and the inlet portion 41a of the housing 41 to retain the filter elements thereon, as illustrated in FIG. 8, the problems raised in the prior art of FIg. 7 will be able to be solved. In this case, however, the number of components is increased and, when a fluid is an organic solvent of halide or a strongly acidic or alkaline fluid, for example, it is required to select a proper material for the shock absorbing member 44, thus leading to increase in cost.
In FIG. 9(A) or FIG. 9(B), if the housing 41 is made of synthetic resin, the ridge 45 or projection 46 can be formed with ease by injection molding. When the housing 41 is made of metal, however, mechanical processing including milling and drilling operations is required, thereby inevitably forming burrs. The burr-galling operation is very cumbersome and brings about increase in cost. The formation of such burrs is a serious problem to be solved not only in the semiconductor industry and electronic industry requiring the smoothness of the inner surface of the housing but also in the food industry and chemical industry. When a fluid exhibits high corrosion, the burrs will corrode and be scattered in the form of minute metallic particles which may pass through the filter medium.
Generally, the filters of the aforementioned types are coupled to pipes through couplers, as illustrated in FIG. 10, by inserting cylindrical coupling portions 47 into the inlet and outlet portions 41a and 41b, welding the inserted cylindrical coupling portions 47 and the inner surface of the housing to form a weld a, and coupling the cylindrical coupling portions 47 to pipes through the couplers. The formation of such weld a will lose the necessary smoothness of the inner surface of the housing.